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Document Name: APHA Method 4500-CL: Standard Methods for the 

Examination of Water and Wastewater 

CFR Section(s): 

Standards Body: American Public Health Association 


For the 

Examination of 
Water and 

: mm 


Prepared and published jointly by: 

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American Water Works Association 

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joint Editorial Board 

Arnold E. Greenberg, APHA, Chairman 

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Andrew D. Eaton, AWWA 

Managing Editor 
Mary Ann H. Franson 

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Copyright © 1 981 by 
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Standard methods for the examination of water and wastewater. 
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4500-CI A. Introduction 

1 . Effects of Chlorination 

The chlorination of water supplies and polluted waters serves 
primarily to destroy or deactivate disease-producing microor- 
ganisms. A secondary benefit, particularly in treating drinking 
water, is the overall improvement in water quality resulting from 
the reaction of chlorine with ammonia, iron, manganese, sulfide, 
and some organic substances. 

Chlorination may produce adverse effects. Taste and odor 
characteristics of phenols and other organic compounds present 
in a water supply may be intensified. Potentially carcinogenic 
chloroorganic compounds such as chloroform may be formed. 
Combined chlorine formed on chlorination of ammonia- or amine- 
bearing waters adversely affects some aquatic life. To fulfill the 
primary purpose of chlorination and to minimize any adverse 
effects, it is essential that proper testing procedures be used with 
a foreknowledge of the limitations of the analytical determina- 

2. Chlorine Forms and Reactions 

Chlorine applied to water in its molecular or hypochlorite form 
initially undergoes hydrolysis to form free chlorine consisting of 
aqueous molecular chlorine, hypochlorous acid, and hypochlor- 
ite ion. The relative proportion of these free chlorine forms is 
pH- and temperature-dependent. At the pH of most waters, 
hypochlorous acid and hypochlorite ion will predominate. 

Free chlorine reacts readily with ammonia and certain nitrog- 
enous compounds to form combined chlorine. With ammonia, 
chlorine reacts to form the chloramines: monochloramine, di- 
chloramine, and nitrogen trichloride. The presence and concen- 
trations of these combined forms depend chiefly on pH, tem- 
perature, initial chlorine-to-nitrogen ratio, absolute chlorine 
demand, and reaction time. Both free and combined chlorine 
may be present simultaneously. Combined chlorine in water sup- 
plies may be formed in the treatment of raw waters containing 
ammonia or by the addition of ammonia or ammonium salts. 
Chlorinated wastewater effluents, as well as certain chlorinated 
industrial effluents, normally contain only combined chlorine. 
HistoricaHyrthe principal analytical problem has been to distin- 
guish between free and combined forms of chlorine. 

3. Selection of Method 

In two separate but related studies, samples were prepared 
and distributed to participating laboratories to evaluate chlorine 
methods. Because of poor accuracy and precision and a high 
overall (average) total error in these studies, all orthotolidine 
procedures except one were dropped in the 14th edition of this 
work. The useful stabilized neutral orthotolidine method was 
deleted from the 15th edition because of the toxic nature of 

: Approved by Standard Methods Committee, 1989. 

orthotolidine. The Jeuco crystal violet (LCV) procedure was 
dropped from the 17th edition because of its relative difficulty 
and the lack of comparative advantages. 

a. Natural and treated waters: The iodometric methods (B and 
C) are suitable for measuring total chlorine concentrations greater 
than 1 mg/L, but the amperometric end point of Methods C and 
D gives greater sensitivity. All acidic iodometric methods suffer 
from interferences, generally in proportion to the quantity of 
potassium iodide (KI) and H ' added. 

The amperometric titration method (D) is a standard of com- 
parison for the determination of free or combined chlorine. It 
is affected little by common oxidizing agents, temperature var- 
iations, turbidity, and color. The method is not as simple as the 
colorimetric methods and requires greater operator skill to obtain 
the best reliability. Loss of chlorine can occur because of rapid 
stirring in some commercial equipment. Clean and conditioned 
electrodes are necessary for sharp end points. 

A low-level amperometric titration procedure (E) has been 
added to determine total chlorine at levels below 0.2 mg/L. This 
method is recommended only when quantification of such low 
residuals is necessary. The interferences are similar to those found 
with the standard amperometric procedure (D). The DPD meth- 
ods (Methods F and G) are operationally simpler for determining 
free chlorine than the amperometric titration. Procedures are 
given for estimating the separate mono- and dichloramine and 
combined fractions. High concentrations of monochloramine in- 
terfere with the free chlorine determination unless the reaction 
is stopped with arsenite or thioacetamide. In addition, the DPD 
methods are subject to interference by oxidized forms of man- 
ganese unless compensated for by a blank. 

The amperometric and DPD methods are unaffected by di- 
chloramine concentrations in the range of to 9 mg CI as CL/L 
in the determination of free chlorine. Nitrogen trichloride, if 
present, may react partially as free chlorine in the amperometric, 
DPD, and FACTS methods. The extent of this interference in 
the DPD methods does not appear to be significant. 

The free chlorine test, syringaldazine (FACTS, Method H) 
was developed specifically for free chlorine. It is unaffected by 
significant concentrations of monochloramine, dichloramine, ni- 
trate, nitrite, and oxidized forms of manganese. 1 

Sample color and turbidity may interfere in all colorimetric 

Organic contaminants may produce a false free chlorine read- 
ing in most colorimetric methods (see II 36 below). Many strong 
oxidizing agents interfere in the measurement of free chlorine 
in all methods. Such interferences include bromine, chlorine 
dioxide, iodine, permanganate, hydrogen peroxide, and ozone. 
However, the reduced forms of these compounds — bromide, 
chloride, iodide, manganous ion, and oxygen, in the absence of 
other oxidants, do not interfere. Reducing agents such as ferrous 
compounds, hydrogen sulfide, and oxidizable organic matter 
generally do not interfere. 

b. Wastewaters: The determination of total chlorine in samples 
containing organic matter presents special problems. Because of 

CHLORINE (RESIDUAL) (4500-CI)/lntroduction 


the presence of ammonia, amines, and organic compounds, par- 
ticularly organic nitrogen, residual chlorine exists in a combined 
state. A considerable residual may exist in this form, but at the 
same time there may be appreciable unsatisfied chlorine demand. 
Addition of reagents in the determination may change these 
relationships so that residual chlorine is lost during the analysis. 
Only the DPD method for total chlorine is performed under 
neutral pH conditions. In wastewater, the differentiation be- 
tween free chlorine and combined chlorine ordinarily is not made 
because wastewater chlorination seldom is carried far enough to 
produce free chlorine. 

The determination of residual chlorine in industrial wastes is 
similar to that in domestic wastewater when the waste contains 
organic matter, but may be similar to the determination in water 
when the waste is low in organic matter. 

None of these methods is applicable to estuarine or marine 
waters because the bromide is converted to bromine and bro- 
mamines, which are detected as free or total chlorine. A pro- 
cedure for estimating this interference is available for the DPD 

Although the methods given below are useful for the deter- 
mination of residual chlorine in wastewaters and treated ef- 
fluents, select the method in accordance with sample composi- 
tion. Some industrial wastes, or mixtures of wastes with domestic 
wastewater, may require special precautions and modifications 
to obtain satisfactory results. 

Determine free chlorine in wastewater by any of the methods 
provided that known interfering substances are absent or appro- 
priate correction techniques are used. The amperometric method 
is the method of choice because it is not subject to interference 
from color, turbidity, iron, manganese, or nitrite nitrogen. The 
DPD method is subject to interference from high concentrations 
of monochloramine, which is avoided by adding thioacetamide 
immediately after reagent addition. Oxidized forms of man- 
ganese at all levels encountered in water will interfere in all 
methods except in the free chlorine measurement of ampero- 
metric titrations and FACTS, but a blank correction for man- 
ganese can be made in Methods F and G. 

The FACTS method is unaffected by concentrations of mono- 
chloramine, dichloramine, nitrite, iron, manganese, and other 
interfering compounds normally found in domestic wastewaters. 

For total chlorine in samples containing significant amounts 
of organic matter, use either the DPD methods (F and G), am- 
perometric, or iodometric back titration method (C) to prevent 
contact between the full concentration of liberated iodine and 
the sample. With Method C, do not use the starch-iodide end 
point if the concentration is less than 1 mg/L. In the absence of 
interference, the amperometric and starch-iodide end points give 
concordant results. The amperometric end point is inherently 
more sensitive and is free of interference from color and turbid- 
ity, which can cause difficulty with the starch-iodide end point. 
On the other hand, certain metals, surface-active agents, and 
complex anions in some industrial wastes interfere in the am- 
perometric titration and indicate the need for another method 
for such wastewaters. Silver in the form of soluble silver cyanide 
complex, in concentrations of 1.0 mg Ag/L, poisons the cell at 
pH 4.0 but not at 7.0. The silver ion, in the absence of the cyanide 
complex, gives extensive response in the current at pH 4.0 and 
gradually poisons the cell at all pH levels. Cuprous copper in 
the soluble copper cyanide ion, in concentrations of 5 mg Cu/L 
or less, poisons the cell at pH 4.0 and 7.0. Although iron and 

nitrite may interfere with this method, minimize the interference 
by buffering to pH 4.0 before adding KI. Oxidized forms of 
manganese interfere in all methods for total chlorine including 
amperometric titration. An unusually high content of organic 
matter may cause uncertainty in the end point. 

Regardless of end-point detection, either phenylarsine oxide 
or thiosulfate may be used as the standard reducing reagent at 
pH 4. The former is more stable and is preferred. 

The DPD titrimetric and colorimetric methods (F and G, re- 
spectively) are applicable to determining total chlorine in pol- 
luted waters. In addition, both DPD procedures and the am- 
perometric titration method allow for estimating monochloramine 
and, dichloramine fractions. Because all methods for total chlo- 
rine depend on the stoichiometric production of iodine, waters 
containing iodine-reducing substances may not be analyzed ac- 
curately by these methods, especially where iodine remains in 
the solution for a significant time. This problem occurs in Meth- 
ods B and D. The back titration procedure (C) and Methods F 
and G cause immediate reaction of the iodine generated so that 
it has little chance to react with other iodine-reducing substances. 

In all colorimetric procedures, compensate for color and tur- 
bidity by using color and turbidity blanks. 

A method (I) for total residual chlorine using a potentiometric 
iodide electrode is proposed. This method is suitable for analysis 
of chlorine residuals in natural and treated waters and wastewater 
effluents. No differentiation of free and combined chlorine is 
possible. This procedure is an adaptation of other iodometric 
techniques and is subject to the same inferences. 

4. Sampling and Storage 

Chlorine in aqueous solution is not stable, and the chlorine 
content of samples or solutions, particularly weak solutions, will 
decrease rapidly. Exposure to sunlight or either strong light or 
agitation will accelerate the reduction of chlorine. Therefore, 
start chlorine determinations immediately after sampling, avoid- 
ing excessive light and agitation. Do not store samples to be 
analyzed for chlorine. 

5. Reference 

1. Cooper, W.J., N.M. Roscher & R.A. Slifer. 1982. Determining 
free available chlorine by DPD-colorimetric, DPD-steadifac (colori- 
metric) and FACTS procedures. J. Amer. Water Works Assoc. 74:362. 

6. Bibliography 

Marks, H.C., D.B. Williams &G.U. Glasgow. 1951. Determination 
of residual chlorine compounds. J. Amer. Water Works Assoc. 43:201. 

Nicolson, N.J. 1965. An evaluation of the methods for determining 
residual chlorine in water, Part 1. Free chlorine. Analyst 90:187. 

Whittle, G.P. & A. Lapteff, Jr. 1973. New analytical techniques for 
the study of water disinfection. In Chemistry of Water Supply, Treat- 
ment, and Distribution, p. 63. Ann Arbor Science Publishers. Ann 
Arbor, Mich. 

Guter, W.J., W.J. Cooper & C.A. Sorber. 1974. Evaluation of ex- 
isting field test kits for determining free chlorine residuals in aqueous 
solutions. J. Amer. Water Works Assoc. 66:38. 



4500-CI B. lodometric Method 

1. General Discussion 

a. Principle: Chlorine will liberate free iodine from potassium 
iodide (KI) solutions at pH 8 or less. The liberated iodine is 
titrated with a standard solution of sodium thiosulfate (Na 2 S 2 3 ) 
with starch as the indicator. Titrate at pH 3 to 4 because the 
reaction is not stoichiometric at neutral pH due to partial oxi- 
dation of thiosulfate to sulfate. 

b. Interference: Oxidized forms of manganese and other oxi- 
dizing agents interfere. Reducing agents such as organic sulfides 
also interfere. Although the neutral titration minimizes the in- 
terfering effect of ferric and nitrite ions, the acid titration is 
preferred because some forms of combined chlorine do not react 
at pH 7. Use only acetic acid for the acid titration; sulfuric acid 
(H 2 S0 4 ) will increase interferences; never use hydrochloric acid 
{hid). See Section A. 3 for discussion of other interferences. 

c. Minimum detectable concentration: The minimum detectable 
concentration approximates 40 jxg CI as Cl 2 /L if 0.01A r Na 2 S 2 3 
is used with a 1000-mL sample. Concentrations below 1 mg/L 
cannot be determined accurately by the starch-iodide end point 
used in this method. Lower concentrations can be measured with 
the amperometric end point in Methods C and D. 

2. Reagents 

a. Acetic acid, cone (glacial), 

b. Potassium iodide, KI, crystals. 

c. Standard sodium thiosulfate, 0.1N: Dissolve 25 g Na 2 S 2 3 - 
5H 2 in 1 L freshly boiled distilled water and standardize against 
potassium bi-iodate or potassium dichromate after at least 2 weeks 
storage. This initial storage is necessary to allow oxidation of 
any bisulfite ion present. Use boiled distilled water and add a 
few milliliters chloroform (CHC1 3 ) to minimize bacterial decom- 

Standardize 0.17V Na 2 S 2 3 by one of the following: 

1) Iodate method — Dissolve 3.249 g anhydrous potassium bi- 
iodate, KH(I0 3 ) 2 , primary standard quality; or 3.567 g KI0 3 
dried at 103 ± 2°C for 1 h, in distilled water and dilute to 1000 
mL to yield a 0.1000/V solution. Store in a glass-stoppered bottle. 

To 80 mL distilled water, add, with constant stirring, 1 mL 
cone H 2 S0 4 , 10.00 mL QAQQQN KH(I0 3 ) 2 , and 1 g KI. Titrate 
immediately with 0.1 A Na 2 S 2 3 titrant until the yellow color of 
the liberated iodine almost is discharged. Add 1 mL starch in- 
dicator solution and continue titrating until the blue color dis- 

2) Dichromate method — Dissolve 4.904 g anhydrous potas- 
sium dichromate, K 2 Cr 2 7 , of primary standard quality, in dis- 
tilled water and dilute to 1000 mL to yield a 0.10007V solution. 
Store in a glass-stoppered bottle. 

Proceed as in the iodate method, with the following exceptions: 
Substitute 10.00 mL 0.1000N K 2 Cr 2 7 for iodate and let reaction 
mixture stand 6 min in the dark before titrating with 0.17V Na 2 S 2 3 

Normality Na 2 S 2 Q 3 


mL Na 2 S 2 3 consumed 

d. Standard sodium thiosulfate titrant, 0.017V or 0.0257V: Im- 
prove the stability of 0.017V or 0.0257V Na 2 S 2 3 by diluting an 

aged 0.17V solution, made as directed above, with freshly boiled 
distilled water. Add 4 g sodium borate and 10 mg mercuric iodide/ 
L solution. For accurate work, standardize this solution daily in 
accordance with the directions given above, using 0.017V or 0,025A f 
iodate or K 3 Cr 2 7 . Use sufficient volumes of these standard 
solutions so that their final dilution is not greater than 1 + 4. 
To speed up operations where many samples must be titrated 
use an automatic buret of a type in which rubber does not come 
in contact with the solution. Standard titrants, O.OIOOA^ and 
0.02507V, are equivalent, respectively, to 354.5 (xg and 886.3 jxg 
CJ as Cl 2 /L00mL. 

e. Starch indicator solution: To 5 g starch (potato, arrowroot, 
or soluble), add a little cold water and grind in a mortar to a 
thin paste. Pour into 1 L of boiling distilled water, stir, and let 
settle overnight. Use clear supernate. Preserve with 1.25 g sal- 
icylic acid, 4 g zinc chloride, or a combination of 4 g sodium 
propionate and 2 g sodium azide/L starch solution. Some com- 
mercial starch substitutes are satisfactory. 

/. Standard iodine, 0.17V; See C3g. 

g. Dilute standard iodine, 0.0282W: See C.3/z. 

3. Procedure 

a. Volume of sample: Select a sample volume that will require 
no more than 20 mL 0.017V Na 2 S 2 3 and no less than 0.2 mL for 
the starch-iodide end point. For a chlorine range of 1 to 10 mg/ 
L, use a 500-mL sample; above 10 mg/L, use proportionately 
less sample. Use smaller samples and volumes of titrant with the 
amperometric end point. 

b. Preparation for titration: Place 5 mL acetic acid, or enough 
to reduce the pH to between 3.0 and 4.0, in a flask or white 
porcelain casserole. Add about 1 g KI estimated on a spatula. 
Pour sample in and mix with a stirring rod. 

c. Titration: Titrate away from direct sunlight. Add 0.0257V or 
0.017V Na 2 S 2 3 from a buret until the yellow color of the liberated 
iodine almost is discharged. Add 1 mL starch solution and titrate 
until blue color is discharged. 

If the titration is made with 0.0257V Na 2 S 2 3 instead of 0.01A, 
then, with a 1-L sample, 1 drop is equivalent to about 50 (xg/L. 
It is not possible to discern the end point with greater accuracy. 

d. Blank titration: Correct result of sample titration by deter- 
mining blank contributed by oxidizing or reducing reagent im- 
purities. The blank also compensates for the concentration of 
iodine bound to starch at the end point. 

Take a volume of distilled water corresponding to the sample 
used for titration in Us 3a-c t add 5 mL acetic acid, 1 g KI, and 
1 mL starch solution. Perform blank titration as in 1) or 2) below, 
whichever applies. 

1) If a blue color develops, titrate with 0.017V or 0.025A' Na 2 S 2 3 
to disappearance of blue color and record result. B (see H 4, 
below) is negative. 

2) If no blue color occurs, titrate with 0.0282^ iodine solution 
until a blue color appears. Back-titrate with 0.017V or 0.0257V 
Na 2 S 2 3 and record the difference. B is positive. 

Before calculating the chlorine concentration, subtract the blank 
titration of 1] 1) from the sample titration; or, if necessary, add 
the net equivalent value of the blank titration of H 2). 

CHLORINE (RESIDUAL) (4500-CI)/lodometric Method 


4. Calculation 

For standardizing chlorine solution for temporary standards: 

(A ± B) x N x 35.45 

mg CI as Ci 2 /mL 

mL sample 

For determining total available residual chlorine in a water 

mg CI as CL/L = 

(A ± B) x N x 35 450 
mL sample 


A = mL titration for sample, 

B = mL titration for blank (positive or negative), and 

N = normality of Na ? S->O v 

5. Precision and Bias 

Published studies 1 - 2 give the results of nine methods used to 
analyze synthetic water samples without interferences; variations 
of some of the methods appear in this edition. More current data 
are not now available. 

6. References 

1. Water Chlorine (Residual) No. 1. 1969. Analytical Reference Service 
Rep. No. 35, U.S. Environmental Protection Agency, Cincinnati, 

2. Water Chlorine (Residual) No. 2. 1971 . Analytical Reference Service 
Rep. No. 40, U.S. Environmental Protection Agency, Cincinnati, 

7. Bibliography 

Lea, C. 1933. Chemical control of sewage chlorination. The use and 
value of orthotolidine test. J. Soc. Chem. Ind. (London) 52:245T. 

American Waterworks Association. 1943. Committee report. Con- 
trol of chlorination. J. Amer. Water Works Assoc. 35:1315. 

Marks, H.C., R. Joiner & F.B. Strandskov. 1948. Amperometric 
titration of residual chlorine in sewage. Water Sewage Works 95:175. 

Strandskov, F.B. , H.C. Marks & D.H. Horchier. 1949. Application 
of a new residual chlorine method to effluent chlorination. Sewage 
Works J. 21:23. 

Nusbaum, I. & L.A. Meyerson. 1951. Determination of chlorine de- 
mands and chlorine residuals in sewage. Sewage Ind. Wastes 23:968. 

Marks, H.C, & N.S. Chamberlin. 1953. Determination of residual 
chlorine in metal finishing wastes. Anal. Chem. 24:1885. 

4500-CI C. lodometric Method II 

1 . Genera! Discussion 

a. Principle: In this method, used for wastewater analysis, the 
end-point signal is reversed because the unreached standard re- 
ducing agent (phenylarsine oxide or thiosulfate) remaining in the 
sample is titrated with standard iodine or standard iodate, rather 
than the iodine released being titrated directly. This indirect 
procedure is necessary regardless of the method of end-point 
detection, to avoid contact between the full concentration of 
liberated iodine and the wastewater. 

When iodate is used as a back titrant, use only phosphoric 
acid. Do not use acetate buffer. 

b. Interference: Oxidized forms of manganese and other oxi- 
dizing agents give positive interferences. Reducing agents such 
as organic sulfides do not interfere as much as in Method B. 
Minimize iron and nitrite interference by buffering to pH 4.0 
before adding potassium iodide (KI). An unusually high content 
of organic matter may cause some uncertainty in the end point. 
Whenever manganese, iron, and other interferences definitely 
are absent, reduce this uncertainty and improve precision by 
acidifying to pH 1.0. Control interference from more than 0.2 
mg nitrite/L with phosphoric acid-sulfamic acid reagent. A larger 
fraction of organic chloramines will react at lower pH along with 
interfering substances. See Section A. 3 for a discussion of other 

2. Apparatus 

For a description of the amperometric end-point detection 
apparatus and a discussion of its use, see D,2#. 

3. Reagents 

a. Standard phenylarsine oxide solution, 0.005 64A/: Dissolve 
approximately 0.8 g phenylarsine oxide powder in 150 mL 0.3A 
NaOH solution. After settling, decant 110 mL into 800 mL dis- 
tilled water and mix thoroughly. Bring to pH 6 to 7 with 6A/ HCl 
and dilute to 950 mL with distilled water. Caution: Severe poi- 
son, cancer suspect agent. 

Standardization — Accurately measure 5 to 10 mL freshly 
standardized 0.0282A7 iodine solution into a flask and add 1 mL 
KI solution. Titrate with phenylarsine oxide solution, using the 
amperometric end point (Method D) or starch solution (see B.2e) 
as an indicator. Adjust to 0.005 64A/ and recheck against the 
standard iodine solution; 1.00 mL = 200 jxg available chlorine. 
(Caution: Toxic — take care to avoid ingestion.) 

b. Standard sodium thiosulfate solution, 0.1N: See B.2c. 

c. Standard sodium, thiosulfate solution, 0.005 64A/: Prepare by 
diluting 0.1A/ Na 2 S 2 3 . For maximum stability of the dilute so- 
lution, prepare by diluting an aged 0.1 N solution with freshly 
boiled distilled water (to minimize bacterial action) and add 10 
mg Hgl 2 and 4 g Na 4 B 4 7 /L. Standardize daily as directed in 
B.2c using 0.005 647V K 2 Cr 2 7 or iodate solution. Use sufficient 
volume of sample so that the final dilution does not exceed 1 + 
2. Use an automatic buret of a type in which rubber does not 
come in contact with the solution. 1.00 mL = 200 |mg available 

d. Potassium iodide, KI, crystals. 

e. Acetate buffer solution, pH 4.0: Dissolve 146 g anhydrous 
NaC 2 H 3 2 , or 243 g NaC 2 H 3 6 2 -3H 2 0, in 400 mL distilled water, 
add 480 g cone acetic acid, and dilute to 1 L with chlorine- 
demand-free water. 



/. Standard arsenite solution, 0.1 N: Accurately weigh a stop- 
pered weighing bottle containing approximately 4.95 g arsenic 
trioxide, As 2 3 . Transfer without loss to a 1-L volumetric flask 
and again weigh bottle. Do not attempt to brush out adhering 
oxide. Moisten As 2 3 with water and add 15 g NaOH and 100 
mL distilled water. Swirl flask contents gently to dissolve. Dilute 
to 250 mL with distilled water and saturate with C0 2 , thus con- 
verting all NaOH to NaHC0 3 . Dilute to mark, stopper, and mix 
thoroughly. This solution will preserve its titer almost indefi- 
nitely. (Caution: Severe poison. Cancer suspect agent.) 


g As 2 Q 3 


g. Standard iodine solution, Q.1N: Dissolve 40 g Kl in 25 mL 
chlorine-demand-free water, add 13 g resublimed iodine, and stir 
until dissolved. Transfer to a 1-L volumetric flask and dilute to 

Standardization — Accurately measure 40 to 50 mL 0.1N ar- 
senite solution into a flask and titrate with 0. 17V iodine solution, 
using starch solutionis indicator. To obtain accurate results, 
insure that the solution is saturated with C0 2 at end of titration 
by passing current of C0 2 through solution for a few minutes 
just before end point is reached, or add a few drops of HC1 to 
liberate sufficient C0 2 to saturate solution. Alternatively stand- 
ardize against Na 2 S 2 3 ; see B.2cl). 

Optionally, prepare O.IOOOjV iodine solution directly as a 
standard solution by weighing 12.69 g primary standard resub- 
limed iodine. Because L may be volatilized and lose from both 
solid and solution, transfer the solid immediately to KI as spec- 
ified above. Never let solution stand in open containers for ex- 
tended periods. 

h. Standard iodine titrate, 0.0282 A/: Dissolve 25 g KI in a little 
distilled water in a 1-L volumetric flask, add correct amount of 
0.17V iodine solution exactly standardized to yield a 0.0282N 
solution, and dilute to 1 L with chlorine-demand-free water. For 
accurate work, standardize daily according to directions in 11 3g 
above, using 5 to 10 mL of arsenite or Na 2 S 2 3 solution. Store 
in amber bottles or in the dark; protect solution from direct 
sunlight at all times and keep from all contact with rubber. 

/. Starch indicator: See B.2e. 

/. Standard iodate titrant, 0.005 64 N: Dissolve 201 .2 mg primary 
standard grade K10 3 , dried for I h at 103°C, or 183.3 mg primary 
standard anhydrous potassium bi-iodate in distilled water and 
dilute to 1 L. 

k. Phosphoric acid solution, H 3 P0 4 , 1 + 9. 

/. Phosphoric acid-sulfamic acid solution: Dissolve 20 g NH 2 S0 3 H 
in 1 L 1 + 9 phosphoric acid. 

m. Chlorine-demand-free water: Prepare chlorine-demand-free 
water from good-quality distilled or deionized water by adding 
sufficient chlorine to give 5 mg/L free chlorine. After standing 
2 d this solution should contain at least 2 mg/L free chlorine; if 
not, discard and obtain better-quality water. Remove remaining 
free chlorine by placing container in sunlight or irradiating with 
an ultraviolet lamp. After several hours take sample, add KI, 
and measure total chlorine with a colorimetric method using a 
nessler tube to increase sensitivity. Do not use before last trace 
of free and combined chlorine has been removed. 

Distilled water commonly contains ammonia and also may 
contain reducing agents. Collect good-quality distilled or deion- 
ized water in a sealed container from which water can be drawn 

by gravity. To the air inlet of the container add an H 2 S0 4 trap 
consisting of a large test tube half filled with 1 + 1 LLS0 4 
connected in series with a similar but empty test tube. Fit both 
test tubes with stoppers and inlet tubes terminating near the 
bottom of the tubes and outlet tubes terminating near the top 
of the tubes. Connect outlet tube of trap containing H 2 S0 4 to 
the distilled water container, connect inlet tube to outlet of empty 
test tube. The empty test tube will prevent discharge to the 
atmosphere of H 2 S0 4 due to temperature-induced pressure 
changes. Stored in such a container, chlorine-demand-free water 
is stable for several weeks unless bacterial growth occurs. 

4. Procedure 

a. Preparation for titration: 

1) Volume of sample — For chlorine concentration of 10 mg/L 
or less, titrate 200 mL. For greater chlorine concentrations, use 
proportionately less sample and dilute to 200 mL with chlorine- 
demand-free water. Use a sample of such size that not more than 
10 mL phenylarsine oxide solution is required. 

2) Preparation for titration — Measure 5 mL 0.005 64/V phen- 
ylarsine oxide or thiosulfate for chlorine concentrations from 2 
to 5 mg/L, and 10 mL for concentrations of 5 to 10 mg/L, into 
a flask or casserole for titration with standard iodine or iodate. 
Start stirring. For titration by amperometry or standard iodine, 
also add excess KI (approximately 1 g) and 4 mL acetate buffer 
solution or enough to reduce the pH to between 3.5 and 4.2. 

b. Titration: Use one of the following: 

1) Amperometric titration — Add 0.02827V iodine titrant in small 
increments from a 1-mL buret or pipet. Observe meter needle 
response as iodine is added: the pointer remains practically sta- 
tionary until the end point is approached, whereupon each iodine 
increment causes a temporary deflection of the microammeter, 
with the pointer dropping back to its original position. Stop 
titration at end point when a small increment of iodine titrant 
gives a definite pointer deflection upscale and the pointer does 
not return promptly to its original position. Record volume of 
iodine titrant used to reach end point. 

2) Colorimetric (iodine) titration — Add 1 mL starch solution 
and titrate with 0.0282/V iodine to the first appearance of blue 
color that persists after complete mixing. 

3) Colorimetric (iodate) titration — To suitable flask or cas- 
serole add 200 mL chlorine-demand-free water and add, with 
agitation, the required volume of reductant, an excess of KI 
(approximately 0.5 g), 2 mL 10% H 3 P0 4 solution, and 1 mL 
starch solution in the order given, and titrate immediately* with 
0.005 64N iodate solution to the first appearance of a blue color 
that persists after complete mixing. Designate volume of iodate 
solution used as A. Repeat procedure, substituting 200 mL sam- 
ple for the 200 mL chlorine-demand-free water. If sample is 
colored or turbid, titrate to the first change in color, using for 
comparison another portion of sample with H 3 P0 4 added. Des- 
ignate this volume of iodate solution as B, 

5. Calculation 

a. Titration with standard iodine: 

mg CI as CL/L 

(A - 5B) x 200 

* Titration may be delayed up to 10 min without appreciable error if H-,P0 4 is not 
added until immediately before titration. 

CHLORINE (RESIDUAL) (4500CI)/Amperometric Titration Method 



A = mL 0.005 64N reductant, 

B = mL 0.0282 N L, and 

C — mL sample. 

b. Titration with standard iodate: 

(A ~ B) x 200 

mg CI as CL/L = 



/I - mL Na : S 2 3 , 

B - mL iodate required to titrate Na 2 S 2 3 , and 

C — mL sample. 

6. Bibliography 
See B.7. 

4500-CI D. Amperometric Titration Method 

1. General Discussion 

Amperometric titration requires a higher degree of skill and 
care than the colorimetric methods. Chlorine residuals over 2 
mg/L are measured best by means of smaller samples or by 
dilution with water that has neither residual chlorine nor a chlo- 
rine demand. The method can be used to determine total chlorine 
and can differentiate between free and combined chlorine. A 
further differentiation into monochloramine and dichloramine 
fractions is possible by control of KI concentration and pH. 

cl Principle: The amperometric method is a special adaptation 
of the polarographic principle. Free chlorine is titrated at a pH 
between 6.5 and 7.5, a range in which the combined chlorine 
reacts slowly. The combined chlorine, in turn, is titrated in the 
presence of the proper amount of KI in the pH range 3.5 to 4.5. 
When free chlorine is determined, the pH must not be greater 
than 7.5 because the reaction becomes sluggish at higher pH 
values, nor less than 6.5 because at lower pH values some com- 
bined chlorine may react even in the absence of iodide. When 
combined chlorine is determined, the pH must not be less than 
3.5 because of increased interferences at lower pH values, nor 
greater than 4.5 because the iodide reaction is not quantitative 
at higher pH values. The tendency of monochloramine to react 
more readily with iodide than does dichloramine provides a means 
for further differentiation. The addition of a small amount of KT 
in the neutral pH range enables estimation of monochloramine 
content. Lowering the pH into the acid range and increasing the 
KI concentration allows the separation determination of di- 

Organic chloramines can be measured as free chlorine, mono- 
chloramine, or dichloramine, depending on the activity of chlo- 
rine in the organic compound. 

Phenylarsine oxide is stable even in dilute solution and each 
mole reacts with two equivalents of halogen. A special amper- 
ometric cell is used to detect the end point of the residual 
chlorine-phenylarsine oxide titration. The cell consists of a non- 
polarizable reference electrode that is immersed in a salt solution 
and a readily polarizable noble-metal electrode that is in contact 
with both the salt solution and the sample being titrated. In some 
applications, end-point selectivity is improved by adding +200 
mV to the platinum electrode versus silver, silver chloride. An- 
other approach to end-point detection uses dual platinum elec- 
trodes, a mercury cell with voltage divider to impress a potential 
across the electrodes, and a microammeter. If there is no chlorine 
residual in the sample, the microammeter reading will be com- 
paratively low because of cell polarization. The greater the re- 

sidual, the greater the microammeter reading. The meter acts 
merely as a null-point indicator — that is, the actual meter reading 
is not important, but rather the relative readings as the titration 
proceeds. The gradual addition of phenylarsine oxide causes the 
cell to become more and more polarized because of the decrease 
in chlorine. The end point is recognized when no further decrease 
in meter reading can be obtained by adding more phenylarsine 

b. Interference: Accurate determinations of free chlorine can- 
not be made in the presence of nitrogen trichloride, NC1 3 , or 
chlorine dioxide, which titrate partly as free chlorine. When 
present, NC1 3 can titrate partly as free chlorine and partly as 
dichloramine, contributing a positive error in both fractions at 
a rate of approximately 0.1%/min. Some organic chloramines 
also can be titrated in each step. Monochloramine can intrude 
into the free chlorine fraction and dichloramine can interfere in 
the monochloramine fraction, especially at high temperatures 
and prolonged titration times. Free halogens other than chlorine 
also will titrate as free chlorine. Combined chlorine reacts with 
iodide ions to produce iodine. When titration for free chlorine 
follows a combined chlorine titration, which requires addition of 
KI, erroneous results may occur unless the measuring cell is 
rinsed thoroughly with distilled water between titrations. 

Interference from copper has been noted in samples taken 
from copper pipe or after heavy copper sulfate treatment of 
reservoirs, with metallic copper plating out on the electrode. 
Silver ions also poison the electrode. Interference occurs in some 
highly colored waters and in waters containing surface-active 
agents. Very low temperatures slow response of measuring cell 
and longer time is required for the titration, but precision is not 
affected. A reduction in reaction rate is caused by pH values 
above 7.5; overcome this by buffering all samples to pH 7.0 or 
less. On the other hand, some substances, such as manganese, 
nitrite, and iron, do not interfere. The violent stirring of some 
commercial titrators can lower chlorine values by volatilization. 
When dilution is used for samples containing high chlorine con- 
tent, take care that the dilution water is free of chlorine and 
ammonia and possesses no chlorine demand. 

See A. 3 for a discussion of other interferences. 

2. Apparatus 

a. End-point detection apparatus, consisting of a cell unit con- 
nected to a microammeter, with necessary electrical accessories. 
The cell unit includes a noble-metal electrode of sufficient surface 
area, a salt bridge to provide an electrical connection without 



diffusion of electrolyte, and a reference electrode of silver-silver 
chloride in a saturated sodium chloride solution connected into 
the circuit by means of the salt bridge. Numerous commercial 
systems are available. 

Keep platinum electrode free of deposits and foreign matter. 
Vigorous chemical cleaning generally is unnecessary. Occasional 
mechanical cleaning with a suitable abrasive usually is sufficient. 
Keep salt bridge in good operating condition; do not allow it to 
become plugged nor permit appreciable flow of electrolyte through 
it. Keep solution surrounding reference electrode free of con- 
tamination and maintain it at constant composition by insuring 
an adequate supply of undissolved salt at all times. A cell with 
two metal electrodes polarized by a small DC potential also may 
be used. (See Bibliography.) 

b. Agitator, designed to give adequate agitation at the noble- 
metal electrode surface to insure proper sensitivity. Thoroughly 
clean agitator and exposed electrode system to remove all chlorine- 
consuming contaminants by immersing them in water containing 
1 to 2 mg/L free chlorine for a few minutes. Add KI to the same 
water and let agitator and electrodes remain immersed for 5 min. 
After thorough rinsing with chlorine-demand-free water or the 
sample to be tested, sensitized electrodes and agitator are ready 
for use. Remove iodide reagent completely from cell. 

c. Buret: Commercial titrators usually are equipped with suit- 
able burets (1 mL). Manual burets are available.* 

d. Glassware, exposed to water containing at least 10 mg/L 
chlorine for 3 h or more before use and rinsed with chlorine- 
demand-free water. 

3. Reagents 

a. Standard phenylarsine oxide titrant: See C.3a. 

b. Phosphate buffer solution, pH 7: Dissolve 25.4 g anhydrous 
KH 2 P0 4 and 34.1 g anhydrous Na 2 HP0 4 in 800 mL distilled 
water. Add 2 mL sodium hypochlorite solution containing 1% 
chlorine and mix thoroughly. Protect from sunlight for 2 d. De- 
termine that free chlorine still remains in the solution. Then 
expose to sunlight until no chlorine remains. If necessary, carry 
out the final dechlorination with an ultraviolet lamp. Determine 
that no total chlorine remains by adding Kl and measuring with 
one of the colorimetric tests. Dilute to 1 L with distilled water 
and filter if any precipitate is present. 

c. Potassium iodide solution: Dissolve 50 g KI and dilute to 1 
L withfreshly boiled and cooled distilled water. Store in the dark 
in a brown glass-stoppered bottle, preferably in the refrigerator. 
Discard when solution becomes yellow.-. 

d. Acetate buffer solution, pH 4: See C.3c. 

in progressively smaller increments until all needle movement 
ceases. Make successive buret readings when needle action be- 
comes sluggish, signaling approach of end point. Subtract last 
very small increment that causes no needle response because of 
overtitration. Alternatively, use a system involving continuous 
current measurements and determine end point mathematically. 
Continue titrating for combined chlorine as described in 11 4c 
below or for the separate monochloramine and dichloramine 
fractions as detailed in lis 4e and 4/. 

c. Combined chlorine: To sample remaining from free -chlorine 
titration add 1.00 mL KI solution and 1 mL acetate buffer so- 
lution, in that order. Titrate with phenylarsine oxide titrant to 
the end point, as above. Do not refill buret but simply continue 
titration after recording figure for free chlorine. Again subtract 
last increment to give amount of titrant actually used in reaction 
with chlorine. (If titration was continued without refilling buret, 
this figure represents total chlorine. Subtracting free chlorine 
from total gives combined chlorine.) Wash apparatus and sample 
cell thoroughly to remove iodide ion to avoid inaccuracies when 
the titrator is used subsequently for a free chlorine determina- 

d. Separate samples: If desired, determine total chlorine and 
free chlorine on separate samples. If sample pH is between 3.5 
and 9.5 and total chlorine alone is required, treat sample im- 
mediately with 1 mL KI solution followed by i mL acetate buffer 
solution, and titrate with phenylarsine oxide titrant as described 
in II 4c preceding. 

e. Monochloramine: After titrating for free chlorine, add 0.2 
mL KI solution to same sample and, without refilling buret, 
continue titration with phenylarsine oxide titrant to end point. 
Subtract last increment to obtain net volume of titrant consumed 
by monochloramine. 

/. Dichloramine: Add 1 mL acetate buffer solution and 1 mL 
KI solution to same sample and titrate final dichloramine fraction 
as described above. 

5. Calculation 

Convert individual titrations for free chlorine, combined chlo- 
rine, total chlorine, monochloramine, and dichloramine by the 
following equation: 

mg CI as Cl 2 /L 

A x 200 

mL sample 


A = mL phenylarsine oxide titration. 

4. Procedure 

a. Sample volume: Select a sample volume requiring no more 
than 2 mL phenylarsine oxide titrant. Thus, for chlorine con- 
centrations of 2 mg/L or less, take a 200-mL sample; for chlorine 
levels in excess of 2 mg/L, use 100 mL or proportionately less. 

b. Free chlorine: Unless sample pH is known to be between 
6.5 and 7.5, add 1 mL pH 7 phosphate buffer solution to produce 
a pH of 6.5 to 7.5. Titrate with standard phenylarsine oxide 
titrant, observing current changes on microammeter. Add titrant 

: Kimax 17110-F. 5 mL. Kimble Products, Box 1035, Toledo. Ohio, or equivalent. 

6. Precision and Bias 

7. Bibliography 

Foulk, C. W. & A.T. Bawden. 1926. A new type of endpoint in elec- 
trometric titration and its application to iodimetry. J. Amer. Chem. 
Soc. 48:2045. 

Marks, H.C. & J.R. Glass. 1942. A new method of determining re- 
sidual chlorine. J. Amer. Water Works Assoc. 34:1227. 

CHLORINE (RESIDUAL) (4500-CI)/DPD Ferrous Titrimetric Method 


Haller, J.F. & S.S. Listek. 1948. Determination of chloride dioxide 
and other active chlorine compounds in water. Anal. Chem. 20:639. 

Mahan, W.A. 1949. Simplified ampero metric titration apparatus for 
determining residual chlorine in water. Water Sewage Works 96:171. 

Kolthoff, I.M. & J.J. Lingane. 1952. Polarography, 2nd ed. Inter- 

science Publishers, New York, N.Y. 
Morrow, J.J. 1966. Residual chlorine determination with dual polar- 

izable electrodes. J. Amer. Water Works Assoc. 58:363. 

4500-CI E. Low-Level Amperometric Titration Method 

1 . General Discussion 

Detection and quantification of chlorine residuals below 0.2 
mg/L require special modifications to the amperometric titration 
procedure. With these modifications chlorine concentrations at 
the 10-jxg/L level can be measured. It is not possible to differ- 
entiate between free and combined chlorine forms. Oxidizing 
agents that interfere with the amperometric titration method (D) 
will interfere. 

a. Principle: This method modifies D by using a more dilute 
titrant and a graphical procedure to determine the end point. 

b. Interference: See 

2. Apparatus 
See D.2. 

3. Reagents 

a. Potassium bi-iodate, 0.002 256N: Dissolve 0.7332 g anhy- 
drous potassium bi-iodate, KH(I0 3 ) 27 in 500 mL chlorine-free 
distilled water and dilute to 1000 mL. Dilute 10.00 mL to 100.0 
mL with chlorine-free distilled water. Use only freshly prepared 
solution for the standardization of phenylarsine oxide, 

b. Potassium iodide, KI crystals. 

c. Low-strength phenylarsine oxide titrant, 0.000 564N: Dilute 
10.00 mL of 0.005 64N phenylarsine oxide (see C.3«) to 100.0 
mL with chlorine-demand-free water (see C.3m). 

Standardization — Dilute 5.00 mL 0.002 256N potassium bi- 
iodate to 200 mL with chlorine-free water. Add approximately 
1.5 g KI and stir to dissolve. Add 1 mL acetate buffer and let 
stand in the dark for 6 min. Titrate using the amperometric 
titrator and determine the equivalence point as indicated below. 

Normality = 0.002256 x SI A 


A = mL phenylarsine oxide titrant required to reach the equiva- 
lence point of standard bi-iodate. 

4. Procedure 

Select a sample volume requiring no more than 2 mL phen- 
ylarsine oxide titrant. A 200-mL sample will be adequate for 
samples containing less than 0.2 mg total chlorine/L. 

Before beginning titration, rinse buret with titrant several times. 
Rinse sample container with distilled water and then with sample. 
Add 200 mL sample to sample container and approximately 1.5 
g KI. Dissolve, using a stirrer or mixer. Add 1 mL acetate buffer 
and place container in end-point detection apparatus. When the 
current signal stabilizes, record the reading. Initially adjust meter 
to a near full-scale deflection. Titrate by adding small, known, 
volumes of titrant. After each addition, record cumulative vol- 
ume added and current reading when the signal stabilizes. If 
meter reading falls to near or below 10% of full-scale deflection, 
record low reading, readjust meter to near full-scale deflection, 
and record difference between low amount and readjusted high 
deflection. Add this value to all deflection readings for subse- 
quent titrant additions. Continue adding titrant until no further 
meter deflection occurs. If fewer than three titrant additions were 
made before meter deflection ceased, discard sample and repeat 
analysis using smaller titrant increments. 

Determine equivalence point by plotting total meter deflection 
against titrant volume added. Draw straight line through the first 
several points in the plot and a second, horizontal straight line 
corresponding to the final total deflection in the meter. Read 
equivalence point as the volume of titrant added at the inter- 
section of these two lines. 

5. Calculation 

mg CI as Cl 2 /L 

A x 200 x N 
B x 0.000 564 

d. Acetate buffer solution, pH 4: See C3e. 


A = mL titrant at equivalence point, 

B = sample volume, mL, and 

TV - phenylarsine oxide normality. 

6. Bibliography 

Brooks, A. S. & G. L. Sbegert. 1979. Low-level chlorine analysis by 
amperometric titration. J. Water Pollut. Control Fed. 51:2636. 

4500-CI F. DPD Ferrous Titrimetric Method 

1 . General Discussion 

a. Principle: A^N-diethyl-p-phenylenediamine (DPD) is used 
as an indicator in the titrimetric procedure with ferrous ammo- 

nium sulfate (FAS). Where complete differentiation of chlorine 
species is not required, the procedure may be simplified to give 
only free and combined chlorine or total chlorine. 
In the absence of iodide ion, free chlorine reacts instantly with 



DPD indicator to produce a red color. Subsequent addition of 
a small amount of iodide ion acts catalytically to cause mono- 
chloramine to produce color. Addition of iodide ion to excess 
evokes a rapid response from dichloramine. In the presence of 
iodide ion, part of the nitrogen trichloride (NCI 3 ) is included 
with dichloramine and part with free chlorine. A supplementary 
procedure based on adding iodide ion before DPD permits es- 
timating proportion of NC1 3 appearing with free chlorine. 

Chlorine dioxide (C10 2 ) appears, to the extent of one-fifth of 
its total chlorine content, with free chlorine. A full response from 
C10 2 , corresponding to its total chlorine content, may be ob- 
tained if the sample first is acidified in the presence of iodide 
ion and subsequently is brought back to an approximately neutral 
pH by adding bicarbonate ion. Bromine, bromamine, and iodine 
react with DPD indicator and appear with free chlorine. 

Addition of glycine before determination of free chlorine con- 
verts free chlorine to unreactive forms, with only bromine and 
iodine residuals remaining. Subtractions of these residuals from 
the residual measured without glycine permits differentiation of 
free chlorine from bromine and iodine. 

b. pH control: For accurate results careful pH control is es- 
sential. At the proper pH of 6.2 to 6.5, the red colors produced 
may be titrated to sharp colorless end points. Titrate as soon as 
the red color is formed in each step. Too low a pH in the first 
step tends to make the monochloramine show in the free-chlorine 
step and the dichloramine in the monochloramine step. Too high 
a pH causes dissolved oxygen to give a color. 

c. Temperature control: In all methods for differentiating free 
chlorine from chloramines, higher temperatures increase the 
tendency for chloramines to react and lead to increased apparent 
free-chlorine results. Higher temperatures also increase color 
fading. Complete measurements rapidly, especially at higher 

d. Interference: The most significant interfering substance likely 
to be encountered in water is oxidized manganese. To correct 
for this, place 5 mL buffer solution and 0.5 mL sodium arsenite 
solution in the titration flask. Add 100 mL sample and mix. Add 
5 mL DPD indicator solution, mix, and titrate with standard 
FAS titrant until red color is discharged. Subtract reading from 
Reading A obtained by the normal procedure as described in 
1! 3al) of this method or from the total chlorine reading obtained 
in the simplified procedure given in 11 3a4). If the combined 
reagent in powder form (see below) is used, first add KI and 
arsenite to the sample and mix, then add combined buffer- 
indicator reagent. 

As an alternative to sodium arsenite use a 0.25% solution of 
thioacetamide, adding 0.5 mL to 100 mL sample. 

Interference by copper up to approximately 10 mg Cu/L is 
overcome by the EDTA incorporated in the reagents. EDTA 
enhances stability of DPD indicator solution by retarding dete- 
rioration due to oxidation, and in the test itself, provides suppres- 
sion of dissolved oxygen errors by preventing trace metal catal- 

Chromate in excess of 2 mg/L interferes with end-point de- 
termination. Add barium chloride to mask this interference by 

High concentrations of combined chlorine can break through 
into the free chlorine fraction. If free chlorine is to be measured 
in the presence of more than 0.5 mg/L combined chlorine, use 
the thioacetamide modification. If this modification is not used, 
a color-development time in excess of I min leads to progressively 

greater interference from monochloramine. Adding thioacet- 
amide (0.5 mL 0.25% solution to 100 mL) immediately after 
mixing DPD reagent with sample completely stops further re- 
action with combined chlorine in the free chlorine measurement. 
Continue immediately with FAS titration to obtain free chlorine. 
Obtain total chlorine from the normal procedure, i.e., without 

Because high concentrations of iodide are used to measure 
combined chlorine and only traces of iodide greatly increase 
chloramine interference in free chlorine measurements, take care 
to avoid iodide contamination by rinsing between samples or 
using separate glassware. 

See A. 3 for a discussion of other interferences. 

e. Minimum detectable concentration: Approximately 18 |xg CI 
as Cl/L. This detection limit is achievable under ideal conditions; 
normal working detection limits typically are higher. 

2. Reagents 

a. Phosphate buffer solution: Dissolve 24 g anhydrous Na 2 HP0 4 
and 46 g anhydrous KH 2 P0 4 in distilled water. Combine with 
100 mL distilled water in which 800 mg disodium ethylenediamine 
tetraacetate dihydrate (EDTA) have been dissolved. Dilute to 
1 L with distilled water and add 20 mg HgCl 2 to prevent mold 
growth and interference in the free chlorine test caused by any 
trace amounts of iodide in the reagents. (Caution: HgCL is 
toxic — take care to avoid ingestion.) 

b. N,N-Diethyl-p-phenylenediamine (DPD) indicator solution: 
Dissolve 1 g DPD oxalate/' or 1 .5 g DPD sulfate pentahydrate,t 
or 1.1 g anhydrous DPD sulfate in chlorine-free distilled water 
containing 8 mL 1+3 H 2 S0 4 and 200 mg disodium EDTA, 
Make up to 1 L, store in a brown glass-stoppered bottle in the 
dark, and discard when discolored. Periodically check solution 
blank for absorbance and discard when absorbance at 515 nm 
exceeds 0.002/cm. (The buffer and indicator sulfate are available 
commercially as a combined reagent in stable powder form.) 
Caution: The oxalate is toxic — take care to avoid ingestion. 

c. Standard ferrous ammonium sulfate (FAS) titrant: Dissolve 
1. 106 g Fe(NH 4 ) 2 (S0 4 ) 2 -6H 2 in distilled water containing 1 mL 
1 + 3 H 3 S0 4 and make up to 1 L with freshly boiled and cooled 
distilled water. This standard may be used for 1 month, and the 
titer checked by potassium dichromate. For this purpose add 10 
mL 1 + 5 H 2 S0 4 , 5 mL cone H 3 P0 4 , and 2 mL 0.1% barium 
diphenylamine sulfonate indicator to a 100-mL sample of FAS 
and titrate with 0.100A/ primary standard potassium dichromate 
to a violet end point that persists for 30 s. The FAS titrant is 
equivalent to 100 |ULg CI as CU/l.OO mL. 

d. Potassium iodide, KI, crystals. 

e. Potassium iodide solution: Dissolve 500 mg KI and dilute 
to 100 mL, using freshly boiled and cooled distilled water. Store 
in a brown glass-stoppered bottle, preferably in a refrigerator. 
Discard when solution becomes yellow. 

/. Potassium dichromate solution: See B.2c2). 

g. Barium diphenylaminesulfonate, 0.1%: Dissolve 0.1 g 
(C 6 H 5 NHC 6 H 4 -4-S0 3 ) 2 Ba in 100 mL distilled water. 

h. Sodium arsenite solution: Dissolve 5.0 g NaAsCX in distilled 
water and dilute to 1 L. (Caution: Toxic— take care to avoid 

* Eastman chemical No. 7102 or equivalent. 

t Available from Gallard-Schlesinger Chemical Mfg. Corp., 584 Mineola Avenue, 

Carle Place, N.Y. 11514, or equivalent. 

CHLORINE (RESIDUAL) (4500-CI)/DPD Colorimetric Method 


/. Thioacetamide solution: Dissolve 250 mg CH 3 CSNH 2 in 100 
mL distilled water. (Caution: Cancer suspect agent. Take care 
to avoid skin contact or ingestion.) 

j. Chlorine-demand-free water: See C.3m. 

k. Glycine solution: Dissolve 20 g glycine (aminoacetic acid) 
in sufficient chlorine-demand-free water to bring to 100 mL total 
volume. Store under refrigerated conditions and discard if cloud- 
iness develops. 

/. Barium chloride crystals, BaCl 2 *2H 3 0. 

3. Procedure 

The quantities given below are suitable for concentrations of 
total chlorine up to 5 mg/L. If total chlorine exceeds 5 mg/L, 
use a smaller sample and dilute to a total volume of 100 mL. 
Mix usual volumes of buffer reagent and DPD indicator solution, 
or usual amount of DPD powder, with distilled water before 
adding sufficient sample to bring total volume to 100 mL. (If 
sample is added before buffer, test does not work.) 

If chromate is present (>2 mg/L) add and mix 0.2 g BaG 2 -2H 2 0/ 
100 mL sample before adding other reagents. If, in addition, 
sulfate is >500 mg/L, use 0.4 g BaCl 2 -2H 2 O/100 mL sample. 

a. Free chlorine or chlor amine: Place 5 mL each of buffer 
reagent and DPD indicator solution in titration flask and mix 
(or use about 500 mg DPD powder). Add 100 mL sample, or 
diluted sample, and mix. 

1) Free chlorine — Titrate rapidly with standard FAS titrant 
until red color is discharged (Reading A). 

2) Monochloramine — Add one very small crystal of KI (about 
0.5 mg) or 0.1 mL (2 drops) Kl solution and mix. Continue 
titrating until red color is discharged again (Reading B). 

3) Dichloramine — Add several crystals KI (about I g) and mix 
to dissolve. Let stand for 2 min and continue titrating until red 
color is discharged (Reading C). For dichloramine concentra- 
tions greater than 1 mg/L, let stand 2 min more if color driftback 
indicates slightly incomplete reaction. When dichloramine con- 
centrations are not expected to be high, use half the specified 
amount of KL 

4) Simplified procedure for free and combined chlorine or total 
chlorine — Omit 2) above to obtain monochloramine and dich- 
loramine together as combined chlorine. To obtain total chlorine 
in one reading, add full amount of KI at the start, with the 
specified amounts of buffer reagent and DPD indicator, and 
titrate after 2 min standing. 

b. Nitrogen trichloride: Place one very small crystal of KI (about 
0.5 mg) or 0.1 mL KI solution in a titration flask. Add 100 mL 
sample and mix. Add contents to a second flask containing 5 mL 
each of buffer reagent and DPD indicator solution (or add about 
500 mg DPD powder direct to the first flask). Titrate rapidly 
with standard FAS titrant until red color is discharged (Reading 

c. Free chlorine in presence of bromine or iodine: Determine 
free chlorine as in % 3a 1). To a second 100-mL sample, add 1 

mL glycine solution before adding DPD and buffer. Titrate ac- 
cording to II 3a\). Subtract the second reading from the first to 
obtain Reading A. 

4. Calculation 

For a 100-mL sample, 1.00 mL standard FAS titrant = 1.00 
mg CI as CL/L. 


NCI., Absent 

NCI : , Present 


Free CI 

Free CI 

B - A 

NH 2 CI 


C - B 


NHCU -I- ! /2NCI, 



Free CI + ^NCF, 

2(N - A) 


NC1 3 

C - N 



In the event that monochloramine is present with NCI., it will 
be included in A', in which case obtain NC1 3 from 2(N~B). 

Chlorine dioxide, if present, is included in A to the extent of 
one-fifth of its total chlorine content. 

In the simplified procedure for free and combined chlorine, 
only A (free CI) and C (total CI) are required. Obtain combined 
chlorine from C — A. 

The result obtained in the simplified total chlorine procedure 
corresponds to C. 

5. Precision and Bias 
See B.5. 

6. Bibliography 

Palin, A.T. 1957. The determination of free and combined chlorine in 

water by the use of diethyl-p-phenylene diamine. J. Amer. Water 

Works Assoc. 49:873. 
Palin, A.T. 1960. Colorimetric determination of chlorine dioxide in 

water. Water Sewage Works 107:457. 
Palin, A.T. 1961. The determination of free residual bromine in water. 

Water Sewage Works .108:461. 
Nicolson, N.J. 1963, 1965, 1966. Determination of chlorine in water. 

Parts 1, 2, and 3. Water Res. Assoc. Tech. Pap. Nos. 29, 47. and 

Palin, A.T. 1967. Methods for determination, in water, of free and 

combined available chlorine, chlorine dioxide and chlorite, bromine. 

iodine, and ozone using diethyl-p-phenylenediamine (DPD). ./. Inst. 

Water Eng. 21:537. 
Palin. A.T. 1968. Determination of nitrogen trichloride in water. J. 

Amer. Water Works Assoc. 60:847. 
Palin, A.T. 1975. Current DPD methods for residual halogen com- 
pounds and ozone in water. J. Amer. Wafer Works Assoc. 67:32. 
Methods for the Examination of Waters and Associated Materials. 

Chemical Disinfecting Agents in Water and Effluents, and Chlorine 

Demand. 1980. Her Majesty's Stationery Oil,, London, England. 

4500-CI G. DPD Colorimetric Method 

1. General Discussion 

a. Principle: This is a colorimetric version of the DPD method 
and is based on the same principles. Instead of titration with 

standard ferrous ammonium sulfate (FAS) solution as in the 
titrimetric method, a colorimetric procedure is used. 

b. Interference: See A. 3 and FAd. Compensate for color and 
turbidity by using sample to zero photometer. Minimize chro- 



mate interference by using the thioacetamide blank correction. 
c. Minimum detectable concentration: Approximately 10 |mg CI 
as CL/L. This detection limit is achievable under ideal conditions; 
normal working detection limits typically are higher. 

2. Apparatus 

a. Photometric equipment: One of the following is required: 

1) Spectrophotometer, for use at a wavelength of 5 15 nm and 
providing a light path of 1 cm or longer. 

2) Filter photometer, equipped with a filter having maximum 
transmission in the wavelength range of 490 to 530 nm and pro- 
viding a light path of 1 cm or longer. 

b. Glassware: Use separate glassware, including separate spec- 
trophotometer cells, for free and combined (dichJoramine) meas- 
urements, to avoid iodide contamination in free chlorine meas- 

3. Reagents 

See F.2a, b, c, d, e, h, i, and ;. 

4. Procedure 

a. Calibration of photometric equipment: Calibrate instrument 
with chlorine or potassium permanganate solutions. 

1) Chlorine solutions — Prepare chlorine standards in the range 
of 0.05 to 4 mg/L from about 100 mg/L chlorine water stand- 
ardized as follows: Place 2 mL acetic acid and 1.0 to 25 ml 
chlorine-demand-free water in a flask. Add about 1 g KI. Meas- 
ure into the flask a suitable volume of chlorine solution. In choos- 
ing a convenient volume, note that 1 mL 0.025N Na 2 S 2 3 titrant 
(see B.2d) is equivalent to about 0.9 mg chlorine. Titrate with 
standardized 0.0257V Na 2 S 2 3 titrant until the yellow iodine color 
almost disappears. Add 1 to 2 mL starch indicator solution and 
continue titrating to disappearance of blue color. 

Determine the blank by adding identical quantities of acid, 
KI, and starch indicator to a volume of chlorine-demand-free 
water corresponding to the sample used for titration. Perform 
blank titration AorB, whichever applies, according to B3d. 

stock solution to 100 mL with distilled water in a volumetric 
flask. When 1 mL of this solution is diluted to 100 mL with 
distilled water, a chlorine equivalent of 1.00 mg/L will be pro- 
duced in the DPD reaction. Prepare a series of KMn0 4 standards 
covering the chlorine equivalent range of 0.05 to 4 mg/L. Develop 
color by first placing 5 mL phosphate buffer and 5 mL DPD 
indicator reagent in flask and adding 100 mL standard with thor- 
ough mixing as described in b and c below. Fill photometer or 
colorimeter cell from flask and read color at 515 nm. Return cell 
contents to flask and titrate with FAS titrant as a check on any 
absorption of permanganate by distilled water. 

Obtain all readings by comparison to color standards or the 
standard curve before use in calculation. 

b> Volume of sample: Use a sample volume appropriate to the 
photometer or colorimeter. The following procedure is based on 
using 10-mL volumes; adjust reagent quantities proportionately 
for other sample volumes. Dilute sample with chlorine-demand- 
free water when total chlorine exceeds 4 mg/L. 

c. Free chlorine: Place 0.5 mL each of buffer reagent and DPD 
indicator reagent in a test tube or photometer cell. Add 10 mL 
sample and mix. Read color immediately (Reading A). 

d. Monochloramine: Continue by adding one very small crystal 
of KI (about 0.1 mg) and mix. If dichloramine concentration is 
expected to be high, instead of small crystal add 0.1 mL (2 drops) 
freshly prepared KI solution (0.1 g/100 mL). Read color im- 
mediately (Reading B). 

e. Dichloramine: Continue by adding several crystals of KI 
(about 0.1 g) and mix to dissolve. Let stand about 2 min and 
read color (Reading C). 

/. Nitrogen trichloride: Place a very small crystal of KI (about 
0.1 mg) in a clean test tube or photometer cell. Add 10 mL 
sample and mix. To a second tube or cell add 0.5 mL each of 
buffer and indicator reagents; mix. Add contents to first tube or 
cell and mix. Read color immediately (Reading N). 

g. Chromate correction using thioacetamide: Add 0.5 mL 
thioacetamide solution (F.2/) to 100 mL sample. After mixing, 
add buffer and DPD reagent. Read color immediately. Add 
several crystals of KI (about 0.1 g) and mix to dissolve. Let stand 
about 2 min and read color. Subtract the first reading from Read- 
ing A and the second reading from Reading C and use in cal- 

mg CI as CI 2 /mL 

(A + B) x N x 35.45 
mL sample 


N - normality of Na 2 S 2 3 , 
A - mL titrant for sample, 

B = mL titrant for blank (to be added or subtracted according to 
required blank titration. See B3d). 

Use chlorine-demand-free water and glassware to prepare these 
standards. Develop color by first placing 5 mL phosphate buffer 
solution and 5 mL DPD indicator reagent in flask and then adding 
100 mL chlorine standard with thorough mixing as described in 
b and c below. Fill photometer or colorimeter cell from flask 
and read color at 515 nm. Return cell contents to flask and titrate 
with standard FAS titrant as a check on chlorine concentration. 

2) Potassium permanganate solutions — Prepare a stock so- 
lution containing 891 mg KMnO 4 /1000 mL. Dilute 10.00 mL 

5. Calculation 


NC1 3 Absent 

NCI3 Present 


Free CI 

Free CI 

B - A 

NH 2 C1 


C - B 


NHC1 2 + ^NCf, 



Free CI + V2NCI3 

2(N - A) 



C - N 



In the event that monochloramine is present with NC1 3 , it will 
be included in Reading /V, in which case obtain NC1 3 from 2{N—B). 

6. Bibliography 
See F.6. 

CHLORINE (RESIDUAL) (4500-CI)/lodometric Electrode Technique 


4500-CI H. Syringaldazine (FACTS) Method 

1 . General Discussion 

a. Principle: The free (available) chlorine test, syringaldazine 
(FACTS) measures free chlorine over the range of 0.1 to 10 
mg/L. A saturated solution of syringaldazine (3,5-dimethoxy- 
4-hydroxybenzaldazine) in 2-propanol is used. Syringaldazine is 
oxidized by free chlorine on a 1:1 molar basis to produce a 
colored product with an absorption maximum of 530 nm. The 
color product is only slightly soluble in water; therefore, at chlo- 
rine concentrations greater than 1 mg/L, the final reaction mix- 
ture must contain 2-propanol to prevent product precipitation 
and color fading. 

The optimum color and solubility (minimum fading) are ob- 
tained in a solution having a pH between 6.5 and 6,8- At a pH 
less than 6, color development is slow and reproducibility is poor. 
At a pH greater than 7, the coior develops rapidly but fades 
quickly. A buffer is required to maintain the reaction mixture 
pH at approximately 6.7. Take care with waters of high acidity 
or alkalinity to assure that the added buffer maintains the proper 

Temperature has a minimal effect on the color reaction. The 
maximum error observed at temperature extremes of 5 and 35°C 
is ± 10%. 

b. Interferences: Interferences common to other methods for 
determining free chlorine do not affect the FACTS procedure. 
Monochloramine concentrations up to 18 mg/L, dichloramine 
concentrations up to 10 mg/L, and manganese concentrations 
(oxidized forms) up to 1 mg/L do not interfere. Trichloramine 
at levels above 0.6 mg/L produces an apparent free chlorine 
reaction. Very high concentrations of monochloramine (>35 mg/ 
L) and oxidized manganese (>:2.6 mg/L) produce a color with 
syringaldazine slowly. Ferric iron can react with syringaldazine; 
however, concentrations up to 10 mg/L do not interfere. Nitrite 
(<250 mg/L), nitrate (<100 mg/L), sulfate (<1000 mg/L), and 
chloride (^1000 mg/L) do not interfere. Waters with high hard- 
ness (^500 mg/L) will produce a cloudy solution that can be 
compensated for by using a blank. Oxygen ddes not interfere. 

Other strong oxidizing agents, such as iodine, bromine, and 
ozone, will produce a color. 

c. Minimum detectable concentration: The FACTS procedure 
is sensitive to free chlorine concentrations of 0.1 mg/L or less. 

2. Apparatus 

Colorimetric equipment: One of the following is required: 

a. Filter photometer, providing a light path of 1 cm for chlorine 
concentrations <1 mg/L or a light path from 1 to 10 mm for 
chlorine concentrations above 1 mg/L; also equipped with a filter 
having a band pass of 500 to 560 nm. 

b. Spectrophotometer, for use at 530 nm, providing the light 
paths noted above. 

3. Reagents 

a. Chlorine-demand-free water: See C.3m. Use to prepare re- 
agent solutions and sample dilutions. 

b. Syringaldazine indicator: Dissolve 1.15 mg 3,5-dimethoxy- 
4-hydroxybenzaldazine* in 1 L 2-propanol. 

c. 2-Propanol: To aid in dissolution use ultrasonic agitation or 
gentle heating and stirring. Redistill reagent-grade 2-propanol 
to remove chlorine demand. Use a 30.5-cm Vigreux column and 
take the middle 75% fraction. Alternatively, chlorinate good- 
quality 2-propanol to maintain a free residual overnight; then 
expose to UV light or sunlight to dechlorinate. Caution: 2- 
Propanol is extremely flammable. 

d. Buffer: Dissolve 17.01 g KH 2 P0 4 in 250 mL water; pH 
should be 4.4. Dissolve 17.75 g Na 2 HP0 4 in 250 mL water; the 
pH should be 9.9. Mix equal volumes of these solutions to obtain 
FACTS buffer, pH 6.6. Verify pH with pH meter. For waters 
containing considerable hardness or high alkalinity other pH 6.6 
buffers can be used, for example, 23.21 g maleic acid and 16.5 
mL 50% NaOH per liter of water. 

e. Hypochlorite solution: Dilute household hypochlorite so- 
lution, which contains about 30 000 to 50 000 mg CI equivalent/ 
L, to a strength between 100 and 1000 mg/L. Standardize as 
directed in GAa\). 

4. Procedure 

a. Calibration of photometer: Prepare a calibration curve by 
making dilutions of a standardized hypochlorite solution (% 3e). 
Develop and measure colors as described in 1] 4b, below. Check- 
calibration regularly, especially as reagent ages. 

b. Free chlorine analysis: Add 3 mL sample and 0.1 mL buffer 
to a 5-mL-capacity test tube. Add 1 mL syringaldazine indicator, 
cap tube, and invert twice to mix. Transfer to a photometer tube 
or spectrophotometer cell and measure absorbance. Compare 
absorbance value obtained with calibration curve and report cor- 
responding value as milligrams free chlorine per liter. 

5. Bibliography 

Bauer, R. & C. Rupe. 1971. Use of syringaldazine in a photometric 
method for estimating "free' 1 chlorine in water. Anal. Chem. 43:421. 

Cooper, W.J., C.A. Sorber & E.P. Meier. 1975. A rapid, free, avail- 
able chlorine test with syringaldazine (FACTS). /. Amer. Water 
Works Assoc. 67:34. 

Cooper, W.J., P.H. Gibbs, E.M. Ott & P. Patel. 1983. Equivalency 
testing of procedures for measuring free available chlorine: amper- 
ometric titration, DPD, and FACTS. /. Amer. Water Works Assoc. 

* Aldrich No. 17. 753-9, Aldrich Chemical Company, Inc., Cedar Knolls, N.J. 
07927, or equivalent. 

4500-CI I. lodometric Electrode Technique 

1. General Discussion 

a. Principle: This method involves the direct potentiometric 
measurement of iodine released on the addition of potassium 

iodide to an acidified sample. A platinum-iodide electrode pair 
is used in combination with an expanded-scale pH meter. 

b. Interference: All oxidizing agents that interfere with other 
lodometric procedures interfere. These include oxidized man- 



ganese and iodate, bromine, and cupric ions. Silver and mercuric 
ions above 10 and 20 mg/L interfere. 

2. Apparatus 

a. Electrodes: Use either a combination electrode consisting 
of a platinum electrode and an iodide ion-selective electrode or 
two individual electrodes. Both systems are available commer- 

b. pHhnillivoU meter: Use an expanded-scale pH/millivolt me- 
ter with 0.1 mV readability or a direct- reading selective ion me- 

3. Reagents 

a . pH 4 b uffer solution: See C . 3 e . 

b. Chlorine-demand-free water: See C.3m. 

c. Potassium iodide solution: Dissolve 42 g KI and 0.2 g Na 2 C0 3 
in 500 mL chlorine-demand-free, distilled water. Store in a dark 

d. Standard potassium iodate 0.002 81/V; Dissolve 0.1002 g 
KICK in chlorine-demand-free, distilled water and dilute to 1000 
mL. Each 1.0 mL, when diluted to 100 mL, produces a solution 
equivalent to 1 mg/L as Cl 2 . 

4. Procedure 

a. Standardization: Pi pet into three 100-mL stoppered volu- 
metric flasks 0.20, .1 . 00, and 5.00 mL standard iodate solution. 
Add to each flask, and a fourth flask to be used as a reagent 
blank, 1 mL each of acetate buffer solution and KI solution. 
Stopper, swirl to mix, and let stand 2 min before dilution. Dilute 
each standard to 100 mL with chlorine-demand-free distilled water. 
Stopper, invert flask several times to mix, and pour into separate 
150-mL beakers. Stir gently without turbulence, using a magnetic 
stirrer, and immerse electrode(s) in the 0.2-mg/L (0.2-mL) stand- 
ard. Wait for the potential to stabilize and record potential in 
mV. Rinse electrodes with chlorine-demand-free water and re- 
peat for each standard and for the reagent blank. Prepare a 

calibration curve by plotting, on semilogarithmic paper, potential 
(linear axis) against concentration. Determine apparent chlorine 
concentration in the reagent blank from this graph (Reading B). 
b. Analysis: Select a volume of sample containing no more 
than 0.5 mg chlorine. Pipet 1 mL acetate buffer solution and 1 
mL KI into a 100-mL glass-stoppered volumetric flask. Stopper, 
swirl and let stand for at least 2 min. Adjust sample pH to 4 to 
5, if necessary (mid-range pH paper is adequate for pH meas- 
urement), by adding acetic acid. Add pH-adjusted sample to 
volumetric flask and dilute to mark. Stopper and mix by inversion 
several times. Let stand for 2 min. Pour into a 150-mL beaker, 
immerse the electrode(s), wait for the potential to stabilize, and 
record. If the mV reading is greater than that recorded for the 
5-mg/L standard, repeat analysis with a smaller volume of sam- 

5. Calculation 

Determine chlorine concentration (mg/L) corresponding to the 
recorded mV reading from the standard curve. This is Reading 
A. Determine total residual chlorine from the following: 

Total residual chlorine = A x 1007V 

where V = sample volume, mL. If total residual chlorine is below 
0.2 mg/L, subtract apparent chlorine in reagent blank (Reading 
B) to obtain the true total residual chlorine value. 

6. Bibliography 

Dimmock, N.A. & D. Midgley. J 981. Determination of Total Residual 
Chlorine in Cooling Water with the Orion 97-70 Ion Selective Elec- 
trode. Central Electricity Generating Board (U.K.) Report RD/L/ 

Jenkins, R.L. & R.B. Baird. 1979. Determination of total chlorine 
residual in treated wastewaters by electrode. Anal. Letters .12:125. 

Synnott, J.C. & A.M. Smith. 1985. Total Residual Chlorine by Ion- 
Selective Electrode — from Bench Top to Continuous Monitor. Pa- 
per presented at 5th International Conf. on Chemistry for Protection 
of the Environment, Leuven, Belgium.